The present invention is an improvement of my earlier patent U.S. Pat. No. 5,500,632, issued to the inventor Joseph G. Halser, III on Mar. 19, 1996 entitled "Wide Band Audio Transformer With Multifilar Winding", which is incorporated herein by reference. The purpose of this improvement is to reduce signal imbalance at high frequencies in wideband audio output transformers having a balanced primary winding and an unbalanced secondary winding.
Audio systems with vacuum tube amplifiers are still commercially available and gaining interest even though most modem audio systems typically use solid state transistors. Nonetheless, many people still prefer vacuum tube amplifiers because they enjoy the sound produced by the vacuum tube amplifiers, because they enjoy the lights of the vacuum tubes, or for other reasons. One type of popular vacuum tube amplifier circuit configuration is the push-pull amplifier circuit.
In a push-pull amplifier circuit, one output vacuum tube amplifies the positive half of an input signal, while another output vacuum tube amplifies the negative half of the input signal. Both halves of the signal are input in the primary winding and ultimately combined in the secondary winding of an output transformer. The output vacuum tubes of the push-pull amplifier circuit drive the primary winding of the output transformer, while the secondary winding provides power to the speaker load typically at high currents and low voltages. A conventional push-pull output transformer comprises three windings wound around a magnetic core; a half primary winding for each half of the input signal, and a secondary winding for the speaker load. The vacuum tube push-pull amplifier circuit requires a wideband iron core output transformer to match the high impedance vacuum tubes of the amplifier to the much lower impedance speaker load. The output transformer is the most limiting and most expensive component of an audio amplifier. The output transformer limits bandwidth, efficiency and causes phase shifts at high and low frequencies which may result in instability of an amplifier utilizing negative feedback.
Most audio output transformers utilize a balanced push-pull primary winding and an unbalanced (i.e., single-ended) secondary winding which matches the high impedance output of a push-pull amplifier circuit to the low impedance of the ground referenced speaker load. Typically in this configuration, the center tap of the primary winding and one end of the secondary winding is connected to AC ground. An important advantage of the single-ended secondary winding is that less taps need to be brought out for different speaker loads. The taps have equal high frequency performance, and utilize all the wire in the secondary winding.
At high frequencies, the AC potential difference between each half primary winding and the secondary winding is unequal causing the effective capacitance between each half primary winding and the secondary winding to be different. This results in a signal imbalance at high frequencies (i.e., the amplitudes of the signals in the two half primary windings are different at high frequencies causing distortions in the secondary winding signal). In contrast, a circuit under balanced conditions has sinusoidal signals with identical amplitudes and shifted in time with respect to each other by identical phase angles.
One way of eliminating the signal imbalance at high frequencies is to provide an output transformer with a balanced primary winding and a balanced secondary winding (i.e., two branches that are electrically alike and symmetrical with respect to a common reference point). However, this eliminates the advantages of using an unbalanced (i.e., single-ended) secondary winding, and ground referenced equipment can no longer be used with the amplifier. A balanced secondary winding also requires two feedback paths, one from the high side of the secondary winding and one from the low side of the secondary winding back to the input. An unbalanced secondary winding only requires one feedback path from the output back to the input because one end of the output is connected to AC ground.
Another way to eliminate the signal imbalance is to provide an amplifier without an output transformer. However, this greatly complicates the amplifier design, and severely compromises performance of the audio system.
Signal imbalance may also be eliminated by adding more insulating material between the primary and secondary windings in an effort to reduce the effective capacitance. This requires substantially more insulating material on the half primary winding with the greatest effective capacitance. Increasing insulating material on only one half primary winding complicates the manufacturing process because the transformer would have different thicknesses of insulating material in different places on the transformer windings. The increase in insulating material also causes wires to be further apart, thus reducing coupling and increasing leakage inductance. It is important to keep the leakage inductance and effective capacitance at a minimum.
Leakage inductance is the result of imperfect magnetic coupling between the primary and secondary windings. The leakage inductance may be reduced by increasing the winding width, minimizing the spacing between windings, or subdividing the primary winding into sections and placing the secondary winding between the subdivided primary windings, as in bifilar or multifilar windings.
Effective capacitance may be reduced by increasing the insulating material or dielectric thickness between windings, reducing the winding width, increasing the number of layers, or avoiding large potential differences between winding sections.
Packaging primary and secondary windings in a multifilar ribbon are known in the art to reduce leakage inductance and effective capacitance as detailed in my earlier patent, U.S. Pat. No. 5,500,632, entitled "Wideband Audio Transformer With Multifilar Winding." In this transformer, high AC voltage potential may exist between adjacent wires of the multifilar ribbon, and the wires must be adequately insulated to withstand these high AC voltage potentials. Also, multifilar ribbon windings may create considerable capacitance between adjacent wires limiting the high frequency performance of the amplifier.
In a transformer with multifilar ribbon windings, each wire has a capacitance with respect to the two wires on each side of it in the same layer, and also with respect to wires in the layers above and below it. While capacitance between the wires in adjacent layers may be reduced by increasing the spacing between layers, this increases the leakage inductance of the transformer as mentioned above.
In order to improve the performance of the output transformer at high frequencies, it is desirable to provide a practical and easy winding scheme that reduces signal imbalance at high frequencies without greatly increasing leakage inductance or effective capacitance.